The present invention relates to railroad cars, and in particular, to structures defining doorway openings in boxcars.
Railway boxcars are made with two general types of doorway closures. The first is a simple sliding door arrangement in which the boxcar door slides along a track into a position that closes a doorway opening, where it is kept in place by latching the door to the car body structure that surrounds the doorway opening. The second type of boxcar door is a "plug door" arrangement in which the boxcar door is mounted on a track and first rolls longitudinally of the car body into a position aligned with the doorway opening, then moves laterally inward into a sealing position in the doorway when a latching mechanism is operated.
For a plug door, the railway boxcar structure that defines the doorway typically provides a sealing surface around the doorway perimeter, facing laterally outward toward the plug door. The plug door typically has a gasket or other sealing device extending along the perimeter of its interior face to press against the sealing surface.
Doorways for some previously-known boxcars with plug door assemblies have had square corners. This construction creates difficulties, as forces carried through the car body structures concentrate in the car structures defining the corners of such doorway openings and, over time, may lead to stress fractures in the car structure at the doorway corners. The sealing surface of the doorway is typically located on a portion of an inner doorpost, where it is vulnerable to being damaged by equipment used to load or unload the car. Such damage also results in stress concentrations leading to cracks which may progress into a side sill of the car.
In response, boxcar manufacturers designed plug door assemblies with rounded corner gussets in the doorways. Because of the curved surfaces of such rounded corners, there is no point at which stresses can concentrate excessively.
Rounded corners, while improving resistance to damage, diminish the available doorway width at the threshold. If the rounded doorway opening is enlarged to leave as much clear width at the threshold as in a square-cornered doorway, then a larger, heavier door is required, necessitating heavier framing structure surrounding the doorway.
What is desired, then, is a way to achieve required usable doorway width without increasing total doorway width, yet avoid stress concentrations in doorway corners.